deploy: Better snf-deploy setup support

Use the setup command to add another Ganeti cluster
to an existing synnefo installation. For instance:

 snf-deploy setup --setup synnefo --cluster ganeti2

For a more fine grained use, try something like:

 snf-deploy setup --setup synnefo --role vmc \
   --node new --cluster ganeti

to setup a specific component/role on a given node of a
given setup and cluster.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
docs: Update snf-deploy docs

..to reflect to latest deploy/vcluster changes.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>